The present invention relates to a material for a constructional finished wallboard and a method of preparing the constructional finished wallboard. The constructional finished wallboard of the invention is of a type so-called Satsuma Nakal-Kirishima Kabe, which uses volcanic ash shirasu (hereinafter referred to simply as shirasu) as a material for a constructional finished wall or plaster-finished wall.
Shirasu is a white sandy sediment, and more specifically, shirasu is a generic name for white porous volcanic ejecta and a secondary sediment derived therefrom, which are widely spread in the southern area of Kyushu of Japan. Shirasu is formed such that when magma having a high temperature is erupted and cooled to give rise to crystallization differentiation, main components of magma, such as SiO.sub.2, Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3, FeO, MgO, CaO, Na.sub.2 O, and K.sub.2 O, are crystallized as minerals through crystallization differentiation. Shirasu comprises about 30 wt. % of crystalline minerals and about 70 wt. % of amorphous volcanic glass. The amorphous volcanic glass has a porous pumice-like structure due to vigorous emission of volatile material of magma and comprises 65 to 73 wt. % of Sio.sub.2, 12 to 16 wt. % of Al.sub.2 O.sub.3, 2 to 4 wt. % of CaO, 3 to 4 wt. % of Na.sub.2 O, 2 to 4 wt. % of K.sub.2 O, and 1 to 3 wt. % of iron compounds. The crystalline minerals include plagioclase in a largest amount, and hypersthene, quartz, augite, magnetite and the like in small amounts.
It has been known that when shirasu is quickly heated, softening of glass contained in the shirasu and evaporation of water contained in the shirasu occur at the same time to give rise to foaming of the shirasu, thereby forming hollow spherical bodies which float on water, so-called shirasu balloons (developed by Kyushu Kogyo Gijutsu Shiken Kenkyusho in 1970). The shirasu balloons are about 30 to 600 .mu.m in size and have features such that they are inexpensive, low in bulk density, high in melting point, low in thermal conductivity, low in dielectric constant, harmless, and free from emission of noxious gas and the like. Recently, the shirasu balloons of 600 to 3,000 .mu.m in size have been produced. As main applications of the shirasu balloons, there may be mentioned a coating material for industrial use, which can be applied in a large thickness because it is lightweight. In recent years, an application have been made largely to a material for constructional use, such as a ceiling board or the like which is light weight and is not required to have so high strength. Further, development of a polymeric composite material using pulverized shirasu is in progress.
A conventional plaster-finished wall and plaster wall as constructional walls include tsuchi kabe (loam-based wall) and shikkui kabe (plaster wall). A loam-based wall comprises clay and sand incorporated therein for prevention of cracking. Further, Tokyo straw or the like is incorporated therein for brown coat or middle coating (naka kabe), and humic dark-colored straw, straw bottom (nemoto wara), Manila hemp, carbon fiber or the like is incorporated therein for scratch coat or basic coating (ara kabe). A plaster wall comprises slaked lime as a main component and, mixed therein, paste, fibers for plaster, straw, paper or the like. Further, diatomaceous earth which is sedimentary earth derived from algae of phytoplanktons is used as a material for a wall substrate, a material for a refractory brick or a material for wall-finishing. As a self-hardening material for a loam-based wall, there may be mentioned lime, dolomite plaster, cement and the like as well as clay. These self-hardening materials may be incorporated into clay to enhance hardening properties.
As stated above, in recent years, studies have been made in the field of construction in utilizing shirasu whose estimated amount is large. However, in most of the above-described conventional techniques, shirasu is processed. Accordingly, there has been no commercially available constructional material which utilizes shirasu in its unprocessed original form to make good use of the abundant resource. In other words, there has been made no effective use of shirasu, which utilizes desirable features inherent in shirasu, as a material for a constructional wallboard, such as plaster-finished wallboard or plaster wallboard.
Further, in view of use of a chemical substance or organic material in the conventional techniques, no substantial measures have been taken with respect to hypersensitivity to an organic compound or with respect to a health-injurious house. Moreover, substantially no use has been made of shirasu as a construction material alternative to sand which has been running short.
In addition, in the conventional techniques, it is required to take special measures with respect to, for example, protection against adverse effect on a human body due to alkalinization during construction, prevention of change of color of wood or development of stain on a wood surface at joints in or between interior posts, veranda, casings, baseboards and the like, prevention of cracks due to treatment of joints between boards as substrates, prevention of peeling by preliminary application of a bond reinforcing agent to surfaces of boards as surface preparation of the boards as substrates.
The present invention has been made in view of the above-mentioned problems inherent in the conventional techniques. It is, therefore, an object of the present invention to provide a material for a constructional finished wallboard, and a method of preparing a constructional finished wallboard, wherein volcanic ash shirasu, which is an abundant natural resource, can be effectively used in its unprocessed original form; mild or rough texture inherent in shirasu can be utilized to appear as a construction material; it can prevent measures with respect to hypersensitivity to an organic compound and with respect to a health-injurious house; shirasu can be used as a constructional material alternative to sand which has been running short; and it can permit energy savings, resource savings, free from pollution, innoxiousness, and cost savings; and the constructional finished wallboard is a substantially inorganic composition and has good spreadability, colorability, fire resistance, fire-proof properties, sound-proof properties, moisture conditioning properties, antibacterial properties (substantially fungi-free properties), heat retaining and insulating properties, deodorizing properties, air permeability and the like.